This invention relates generally to a Coin Analyzer System for determining whether a coin is of a particular denomination and more particularly to a sensor configuration and detection circuitry for such a coin analyzer system. The invention is particularly adapted to determining whether a coin is one of a plurality of particular denominations.
U.S. Pat. No. 4,905,814 issued to the present inventor and Robert Rollins for a COIL CONFIGURATION FOR ELECTRONIC COIN TESTER AND METHOD OF MAKING, addressed the problems associated with known coils used to generate the magnetic fields in prior art coin acceptance devices. The known coil configurations generate generally doughnut shaped flux patterns. The coins' attenuation characteristics on the field as it passes through the field varies according to the coin's longitudinal and lateral position in relationship to the coil. As a result, coin paths had to be devised which caused the coin to be precisely positioned as it passed the coil. This usually resulted in significant slowing of the rate of travel of the coin and limited the range of coin sizes that could be successfully accepted with one coil.
The solution proposed in the '814 patent was to generate a magnetic flux normal to the face of a coin throughout the coin's diameter, regardless of the coins longitudinal position within the slot, by generating a magnetic flux pattern that is constant throughout the slot's longitudinal axis. This is accomplished in the '814 patent by providing coils wound in a loop on opposite sides of the coin slot generally parallel the longitudinal walls of the slot. The coils are wound in a manner that leaves a central gap and, thereby, purportedly generates magnetic flux lines that are normal to all points of the longitudinal walls.
The coil configuration of the'814 patent, however, does not operate practically in a commercial environment. The opposing coil loops are not capable of generating sufficient flux density to provide adequate detection-signal strength. Furthermore, the purportedly uniform field normal to the longitudinal walls is easily distorted by surrounding metal surfaces within the coin acceptor assembly, thus negating the intended beneficial effects of the sensor configuration. Attempts at correcting the surrounding-metal susceptibility, such as copper shielding, only tended to produce greater field distortions.
Another problem with prior art coin acceptance devices is the ability to develop adequate detection signal levels in order to provide greater discrimination between various coin denominations and between real and counterfeit coins. Various attempts have been made to improve the levels of the detection signals. For example, in U.S. Pat. No. 4,469,213 issued to Raymond Nicholson and the present inventor for a COIN DETECTOR SYSTEM, a spiked signal source is provided, composed of a square wave voltage source and means for differentiating the square wave to produce a spiked signal containing a plurality of frequencies, ranging from the oscillator frequency of 17 kilohertz and multiples, or harmonics, thereof. While the intent of the '213 patent was to supply multiple frequencies in order to provide greater discrimination among various types of coins, the performance was marginal at best. The frequency spectrum was only sparsely populated and resulted in a resonant pulse being produced by the interaction between the primary oscillator frequency, or a low order harmonic thereof, and the reactance of the testing coils. This resonant pulse dominated the other frequencies in the spectrum and, thereby, eliminated most of the beneficial effect of the mixture of frequencies.
Other approaches to improving detection signal levels included placing the sensing coil in an oscillator circuit and measuring variation in phase angle and amplitude of oscillation output caused by the attenuation of the magnetic field as a test coin passes the sensing coil. Such an approach is suggested in U.S. Pat. No. 4,574,936, in which it is suggested that, by measuring multiple parameters, the ability to discriminate is improved. However, the difficulty experienced by inadequate detection signal levels is only marginally improved by monitoring multiple such signals. Other problems experienced by prior art detection circuits include high susceptibility to temperature variations and to changes in the component values with aging, requiring large acceptance windows to avoid repeated rejection of authentic coins.